Heat exchange coil

ABSTRACT

Aluminum tubes particularly useful in forming heat exchange coils are joined together by first flaring and necking respective tube ends to define tapered mating surfaces to insure maximum wetting of both surfaces by the epoxy which bonds the tubes together when the epoxy coated male tube is inserted axially within the female tube.

United States Patent 11 1 Dreksler HEAT EXCHANGE COIL [76] Inventor:Moshe Y. Dreksler, RR 2, Locust Rd., Harwinton, Conn.

[22] Filed: Mar. 19, 1971 [21] App]. No.: 125,988

[52] US. Cl. 165/150; 165/151; 165/178; 285/DIG. 16; 285/157 [51] Int.Cl F28f 9/08 [58] Field of Search 16 5/143, 144, 172, 173, 165/178;285/D1G. 16, 157, 287, 332

[56] References Cited UNITED STATES PATENTS 1,901,820 3/1933 Parker285/287 2,105,751 l/l938 Money 165/144 X 2,498,831 2/1950 Veitch285/D1G. 16 3,124,874 3/1963 Woolley 285/DIG. 16

[ Apr. 15, 1975 3,430,692 3/1969 Karmazin 165/178 FOREIGN PATENTS ORAPPLICATIONS l,550,596 11/1968 France 285/157 Primary Examiner-CharlesJ. Myhre Assistant Examiner-Theophil W. Streule, Jr. Attorney, Agent, orFirmSughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT Aluminum tubesparticularly useful in forming heat exchange coils are joined togetherby first flaring and necking respective tube ends to define taperedmating surfaces to insure maximum wetting of both surfaces by the epoxywhich bonds the tubes together when the epoxy coated male tube isinserted axially within the female tube.

3 Claims, 6 Drawing Figures P. I mglii j PATENTEDAFR I 5 195 INVENTORMOSHE Y. DREKSLER g]EEC? HEAT EXCHANGE con.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to the formation of high integrity joints betweenaluminum tubes and, more particularly, to the formation of a highintegrity epoxy joint between tubular aluminum members particularlyuseful in the refrigeration field.

2. Description of the Prior Art Heat exchange coils of the fin and tubetype havebeen manufactured in the past by utilizing thin, extruded tubesof metals having high thermal conductivity, in which case sections oftubes, after insertion through holes within a plurality oflongitudinally spaced fins, are coupled together to effect a serpentineflow path through the radiating fins and may in turn, have threadedfittings and the like coupled to the extreme ends of the serpentine coilfor attachment to a liquid source and drain. In the past, heatexchangers of this type have been formed of either copper or aluminumtubes in which case the joints between tube sections or between thetubes and the end fittings were formed generally by various brazingprocesses. Not only is the assembly of aluminum parts and the subsequentbrazing, time consuming and therefore relatively costly and complex. butthe brazing operation is highly critical with low production rates dueto the need for complete rinse of the brazing flux, and a relativelyhigh percentage of leaks occurring at the brazed joints. The presence ofone or more leaks requires rebrazing of the assembly in the leak area toeliminate the same. While rebrazing and soldering allows some of theunits to be made leak-free, certain assemblies have to be discarded,either because of the impossibility to reach the area of the leak or tosuccessfully eliminate the same.

SUMMARY OF THE INVENTION The present invention is directed to a methodof forming a high integrity adhesive joint between tubular aluminummembers and, in particular, to an aluminum heat exchanger coil employingepoxy adhesive joints between tubular aluminum components. The methodinvolves flaring the ends of one tubular member to define an outwardlytapering female mating surface portion and correspondingly necking thecoupling end of the other tubular member to define a correspondinginwardly tapered female mating surface portion. An epoxy adhesive issupplied to at lease one of the mating surface portions, and the maletubular member is axially inserted into the female member until theepoxy uniformly wets both mating surfaces, whereupon the epoxy is curedto effect the high integrity bond. Preferably, the axial insertion iscompleted when a small bead of epoxy is uniformly extruded completelyaround the periphery of the exposed edge of the flared female tubularmember.

The invention is particularly applicable to a heat exchange coil of thetype which includes a plurality of adjacently positioned hairpin-shapedtubes, commonly supported by a plurality of longitudinally spaced heatexchange fins, with the tubes extending through holes formed within thefins. The ends of two separate tubes are joined by a U-shaped returnbend tube to effect a serpentine configuration to the aluminum tube heatexchanger. The improvement lies in flaring the ends of thehairpin-shaped tubes and necking the ends of the return bend tubes orvice versa prior to applying an epoxy adhesive bond between the coplanarmating surfaces of the tube ends to effect a high integrity seal bycuring the epoxy subsequent to coupling of the return bends to theexposed ends of the hairpin-shaped tubes. The extreme tube ends of theassembly may carry threaded couplings which in turn have cooperatingtapered surfaces facing the tube ends for achieving a high integrityepoxy seal therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a heatexchanger coil incorporating the high integrity epoxy joint betweentubular aluminum members forming components of the same.

FIG. 2 is a side elevation of the embodiment of FIG. 1.

FIGS. 3 and 4 are exploded sectional views of portions of the heatexchanger coil of FIG. 1 illustrating how the epoxy joints are to becompleted by the method of the present invention.

FIGS. 5 and 6 are sectional views of the coil portions of FIGS. 3 and 4,respectively, after curing of the epoxy joints.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention isexemplified in a preferred embodiment in terms of the heat exchange coil10, FIG. I, which comprises essentially a plurality of longitudinallyspaced outer fins in the form of rectangular plates 12 of aluminum orother high thermal conductive material, the fins having circularopenings 14 at laterally spaced positions which receive the straightportions 16 of, in this case, a pair of hairpin aluminum tubes 18. Thestraight tube portions 16 are joined by a semicircular connectingportion 20 and the hairpin tubes 18 are joined at one end by a U-shapedreturn bend tube 22 and define with tubes 18, joints indicated generallyat 24. The four tube single row coil assembly 12 therefore consistsessentially of the two hairpin tubes 18 and the return bend tube 22. Theoutside straight tube portions 16 terminate at the same end of theassembly 12 as the return bend tube 22 in a pair of threaded tubularaluminum couplings 26 and the straight tube sections 16 define withcouplings 26 joints 28. The aluminum tubes 18 and 22 are preferablyformed of 3003 Alluminum Alloy or the equivalent, of zero temper,nominal three-quarter inch OD, 0.028 inch wall thickness and beingformed to accommodate the return bend tube 22 of the same material andsize. The joints 24 between the return bend tube 22 and the hairpintubes 18 are in the form of a taper of wedge-like configuration, withthe return bend tube 22 in this case forming the female part. Theresulting overlap wedgejoint at 24 is covered with a single part epoxyhightemperature structural adhesive such as Type 2214 taper outwardly atthe same angle and extend axially the same length as the neck portions30 of the tube ends to be mated therewith. Preferably, in forming boththe flared and neck ends of both tubes 18 and 22, a mechanicaldeformation or swaging process may be employed with respect to bothmating surfaces, to create longitudinal surface indentations or,scratches constituting an axially roughened section which materiallyincreases the wetability between the epoxy 34 which is applied to eitherthe male tapered surface portion 30 or the female tapered portion 32 oftubes 18 and 22 respectively. Alternatively, the epoxy 34 may be appliedto both surfaces after degreasing, as mentioned previously.

Coupling of the extreme of the outside tube ends to aluminum tubularcouplings 26, is achieved in an identical manner. In this case, however,the couplings which consist of cast aluminum alloy, include a threadedbore36 within a flange area 38, and the tubular fitting 26 is providedwith an outwardly flared or tapered counter-bore 40 which is also at anangle of approximately to the tube axis. Thus, the tapered bore 40 actsas the female mating surface for the necked terminal ends 30 of tubes18. Again, either mating surface 40 or surface 30 may have appliedthereto either a coating of epoxy as at 34 or both surfaces may haveapplied coatings prior to assembly.

Reference to FIGS. 5 and 6 indicates portions of a completed assemblyafter curing. It is noted that during the axial insertion of the malemating surfaces 30 into the respective female surfaces 24 of the returnbend tube 22 and 40 of the tubular end fittings 26, the insertionterminates when there is a full wetting of the surfaces by the epoxywhich may be visually observed by the extrusion ofa thin bead 34' ofepoxy about the edge 42 of the return bend tube 22 and the outer edge 44of each of the threaded fittings 26. Once assembled in this manner,curing is achieved at 250 F. for a time period in excess of 50 minutes.The completed assembly may then be tested for leaks and any joints whichare found leaking are repaired in a simple manner by applying heat tothe zone of the epoxy until it burns which is above approximately 600 F.for the 2214 high temperature structural epoxy adhesive. At thistemperature, the aluminum tubing and the aluminum cast fittings 26 areunaffected. The return bend tube 22 and the fittings 26 are removedafter the epoxy burns and the charred exposed epoxy is cleaned eithermechanically or chemically. After the burned epoxy is removed, theparent metal-is exposed and is cleaned, permitting assembly in themanner of FIG. 1 involving normal preparation, that is, degreasing,epoxy application, return bend and fitting assembly and curing. Ifnecessary a chemical remover and a separate cleaner may be used underthe trade names Cee Bee and product designations A227D and 21Drespectively or equivalents. Epoxy may be cleaned effectively byemersing the coil ends for approximately one hour in the A227D chemicalremover.

From the above, it is readily evident in a simplified two-tube heatexchanger coil, a single hairpin tube may be employed with fittings suchas fittings 26 at the free ends thereof and coupled by the epoxyjointing techniques set forth above. Further, multi-tube coils may bemanufactured without the necessity of the fittings 26 with the ends ofthe outside tubes being connected directly to piping, hoses or the likecarrying oil, liquid or gaseous refrigerant or the like.

From the above description it is readily apparent that an epoxy joint ofaluminum tubes or tubular members by the improved method has manyadvantages. The assembly of the invention allows positive positioning ofthe parts to be joined in terms of the area which is to be epoxy bondedand at the same time asures the complete wetting of the mating surfacesby the bond material without scraping off any of the epoxy due to theaxial movement of the tubular members into position for joining by theepoxy bond. This is to be contrasted to conventional joining of tubes byepoxy bond or otherwise in which either the inside of the female tube orthe outside of the male tube is first coated with the bonding material,then, if parallel surfaces carry the coating material during thetelescoping of the male member within the female member, there is atendency for the bonding material to be scraped from the telescopingsurfaces. This not only creates voids, inclusions, or pockets devoid ofbonding material, but necessitates attempting to rectify such asituation which to date have been ineffective to repair the damage. Inone attempt, the application of suction to one of the tubes to cause thebonding material to be pulled in the direction of suction application,still prevents complete circumferential wetting of the mating surfacesand does not materially better the uniformity in thickness of thebonding material. In fact, it has a tendency to move the materialaxially rather than circumferentially and thus the voids created byaxial insertion of one parallel tube surface within another are noteliminated by this approach.

To the contrary, the method assures complete wetting of the matingsurfaces by the bond material without any scraping, since axial shiftingof the male member with respect to the female is terminated when a smallbead of epoxy is exposed at the outer edge of the female member which isindicative of full circumferential wetting along the complete matingtapered surfaces. The reverse tapering of matched mating surfacesinsures uniformity in thickness and a good bond without voids,inclusions or pockets. Further, even if there is incomplete cleaningprior to the application of the epoxy and subsequent curing, there isstill an excellent chance that a completely sealed joint is formed andthe manufactured part will not have to be discarded or reprocessed byburning out the epoxy, recleaning and reapplication and curing of theepoxy.

With the illustrated coil, there is in its most simple form the assemblyof a single return bend tube as a fitting which in itself is simple inview of the joint configuration. The male taper of the tube and thefemale taper of the return bend provide a self-aligning relationshipwhen mated to insure proper position and preventing cocking of one tubewith respect to the other. The mating surfaces are presumed to provide aclose gap with minor variances. Preferably, the surfaces are roughenedpurposely during mechanical deformation to provide the flare and thenecking and thus assure maximal effectiveness of the bond.

In fact, the total mated surface area provides a lap joint which has asheer strength in excess of the tube and where the epoxy joint isprepared and cured properly, destructive pull tests indicate thatfailure occurs not at the joint but at adjacent sections of the tube.The coils of the illustrated type have been employed in coolinglubricating oils circulating through the tubes at 250 psig and 250 F.without problem.

When contrasted to brazed joint copper heat exchanger in terms of theillustrated coil assembly, the economic return employing the method ofthe present invention is startling. The cost of an equivalent epoxyjoint between aluminum tubular members may be as low as 1/10 of thoseinvolving a brazed joint between copper tubes and as low as 1/100 of theconventional comparative brazed joint between aluminum tubular members.Further, tests conducted on epoxy joint, aluminum tube heat exchangersof the type illustrated in FIGS. 1 and 2 indicate seal failure of lessthan onetenth of 1 percent as contrasted to initial seal failure of ashigh as 5 percent for the equivalent copper brazed joint copper tubeheat exchangers. These coils were tested at said temperature andpressure by imparting vibrations to the coil structure and keeping atthe natural frequency for an hour without any apparent weakening or bondleaks.

What is claimed is:

1. In a heat exchange coil of the type including a plurality ofadjacently positioned first hairpin shaped aluminum tubes carryinginternally, a high pressure fluid, and being commonly supported by aplurality of longitudinally spaced heat exchange fins with the straightportions of said tubes extending through holes within the fins and withthe ends of two separate tubes being joined by a second, U-shaped returnbend aluminum tube, the improvement wherein: the ends of one of saidtubes are flared and the ends of said other tubes are necked to defineaxially opposed tapered mating surfaces, and wherein said tubes arecoupled together solely by a uniform thickness epoxy adhesive betweensaid mating surfaces to complete a high integrity, fluid seal bondtherebetween of sufficient strength to prevent longitudinal separationof the return bend tube from the straight portions of the tubes joinedthereto due to the high fluid pressure acting directly on said returnbend tube and creating a resultant force tending to longitudinallyseparate the return bend tube from said hairpin tubes.

2. The heat exchange coil as claimed in claim 1, wherein said firsttubes are two in number and are coupled together by a second return bendtube and wherein said heat exchange coil further includes threadedaluminum tubular fittings having complementary tapered mating surfacesepoxy bonded to the outside ends of said respective first tubes inidentical fashion to that of said return bend tube and acting as thesole means for coupling said fittings to said said first tubes.

3. The heat exchange coil assembly as claimed in claim 1, wherein saidfirst tubes terminate in male neck portions defining inwardly taperedmating surfaces and said second return bend tube terminates in flaredportions defining corresponding outwardly tapering mating surfaces andsaid assembly further includes threaded tubular aluminum fittingscoupled to the outside ends of respective first tubes, said threadedtubular aluminum fittings each including an axially tapered bore whichmates with a necked end of said first tube with a uniform thicknessepoxy adhesive bonding the mating surfaces therebetween, and acts as thesole means for coupling said fittings to the outside ends of respectivefirst tubes.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,877,518

DATED April 15, 1975 INVENTOR(S) I Moshe Yo Dreksler It is certifiedthat error appears in the above-iden tified patent and that said LettersPatent i g are hereby corrected as shown below:

The term of this patent subsequent to August 1 3, I991,

has been disclaimedo Q I I Q I I T Signed and Scaled this] Twenty-fourth0f Augu stjl916 [SEAL] Arrest."

RUTH c.- MASON a c. MARSHALL mum A ffite Commissioner ufParems andTrademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,877,518 Dated Agril 1; 1975 Inventor(s) Moshe ksler It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the first page, after the name of the inventor, please insert thename of the assignee:

- Dunham-Bush, Inc.

West Hartford, Conn.

Signed and sealed this 24th day of June 1975.

(SEAL) fittest:

C. MARSHALL DANN RUTH C. M11305 Commissioner of Patents attestingOfficer and Trademarks FORM F0405) (10-693 USCOMM-DC 0O376-P69 U.$.GOVERNMENT PRINTlNG OFFICE I I9! 0-366-33L

1. In a heat exchange coil of the type including a plurality ofadjacently positioned first hairpin shaped aluminum tubes carryinginternally, a high pressure fluid, and being commonly supported by aplurality of longitudinally spaced heat exchange fins with the straightportions of said tubes exteNding through holes within the fins and withthe ends of two separate tubes being joined by a second, U-shaped returnbend aluminum tube, the improvement wherein: the ends of one of saidtubes are flared and the ends of said other tubes are necked to defineaxially opposed tapered mating surfaces, and wherein said tubes arecoupled together solely by a uniform thickness epoxy adhesive betweensaid mating surfaces to complete a high integrity, fluid seal bondtherebetween of sufficient strength to prevent longitudinal separationof the return bend tube from the straight portions of the tubes joinedthereto due to the high fluid pressure acting directly on said returnbend tube and creating a resultant force tending to longitudinallyseparate the return bend tube from said hairpin tubes.
 2. The heatexchange coil as claimed in claim 1, wherein said first tubes are two innumber and are coupled together by a second return bend tube and whereinsaid heat exchange coil further includes threaded aluminum tubularfittings having complementary tapered mating surfaces epoxy bonded tothe outside ends of said respective first tubes in identical fashion tothat of said return bend tube and acting as the sole means for couplingsaid fittings to said said first tubes.
 3. The heat exchange coilassembly as claimed in claim 1, wherein said first tubes terminate inmale neck portions defining inwardly tapered mating surfaces and saidsecond return bend tube terminates in flared portions definingcorresponding outwardly tapering mating surfaces and said assemblyfurther includes threaded tubular aluminum fittings coupled to theoutside ends of respective first tubes, said threaded tubular aluminumfittings each including an axially tapered bore which mates with anecked end of said first tube with a uniform thickness epoxy adhesivebonding the mating surfaces therebetween, and acts as the sole means forcoupling said fittings to the outside ends of respective first tubes.